1) New inhibitor X-ray data have been measured for the aspartyl proteinase from Rhizopus chinensis. A mechanism of action has been proposed. 2) The crystal structure of tryptophan synthase from Salmonella typhimurium has been determined at 2.3A resolution and is currently being refined. Preliminary analysis of the structure accounts for the behavior of many mutants. It also shows how such a double enzyme system can be catalytically advantageous.